C 55.2: AU 6
Pennsylvania State University
OCT 3 1 1996
U.6. Depository Copy
Space Environment Center
325 Broadway, Boulder, CO 80303-3326
he beautiful and often eerie curtains
of light in the night-time sky known
as aurora have been enjoyed by
people for millennia. Called the au-
rora borealis or "northern lights" (fig. 1), auro-
ra also occur in the Southern Hemisphere and
are called the aurora australis.
Many legends, myths and superstitions have re-
volved around the aurora throughout the histo-
ry of mankind. The early dragon legends of
China and Europe are said to have originated
from the aurora. Some cultures have regarded
the sighting of the aurora as a sign of royal
birth; to others it suggests ghosts of the dead or
Figure 7.— Aurora borealis taken in the Copper River Delta, Alaska; ®1990 by Dave Parkhurst, Alaska Naturally.
the precursor for war. The Eskimos of North
America believed that if you whistled at the au-
rora it would sweep down and take you from the
earth; by clapping your hands you could force
it to retreat.
Magnets Are the Key
The origin of the aurora is 93 million miles ( 149
million km) from Earth at the Sun. Energetic
particles from the Sun are carried out into space
along with the ever-present hot solar wind. This
wind sweeps supersonically toward Earth
through interplanetary space at speeds ranging
from 300 to over 1000 km per second, carrying
with it the solar magnetic field. The solar wind
distorts the Earth's magnetic field to create the
comet-shaped magnetosphere (fig. 2).
The terrestrial magnetic shield acts as a barrier,
protecting the Earth from energetic particles
and radiation in the hot solar wind. Most of
these energetic particles are deflected around
the Earth by the magnetosphere, but some get
trapped. Electrons trapped in the Earth's mag-
netic field are accelerated along the magnetic
field toward the polar regions and then strike
the atmosphere to form the aurora.
m J Auroral oval
fl .-». ^
Figure 3. Energetic electrons spiral down the geo-
magnetic field lines towards the polar regions, strik-
ing the upper atmosphere, resulting in the display of
The particles, which stream down the magnetic
field of the Earth, reach the neutral atmosphere
in a rough circle called the auroral oval. This
circle, or annulus, is centered over the magnetic
pole and is around 3000 km in diameter during
quiet times. The annulus grows larger when the
magnetosphere is disturbed. The location of the
auroral oval is generally found between 60 and
70 degrees north and south latitude (fig. 3).
Figure 2. — A "side view" of the Earth and magnetosphere showing some of the important regions.
Auroral features come in many shapes and
sizes. Tall arcs and rays start brightly 100 km
above the Earth's surface and extend upward
along its magnetic field for hundreds of km.
These arcs or curtains can be as thin as 100 me-
ters while extending from horizon to horizon.
Auroral arcs can nearly stand still and then, as
though a hand has been run along a tall curtain,
the aurora will begin to dance and turn. After
midnight, the aurora can take on a patchy ap-
pearance and the patches often blink on and off
once every 10 seconds or so until dawn.
Most of the auroral features are greenish yellow
but sometimes the tall rays will turn red at their
tops and along their lower edge. On rare occa-
sions, sunlight will hit the top part of the auroral
rays creating a faint blue color. On very rare oc-
casions (once every 10 years or so) the aurora
can be a deep blood red color from top to bot-
tom. In addition to producing light, the energet-
ic auroral particles deposit heat. The heat is dis-
sipated by infrared radiation or transported
away by strong winds in the upper atmosphere.
The Chemistry of the Aurora
The aurora is caused by the interaction of high-
energy particles (usually electrons) with neu-
tral atoms in the Earth's upper atmosphere.
These high-energy particles can "excite" (by
collisions) valence electrons that are bound to
the neutral atom. The excited electrons can then
return to their initial, lower energy state, and in
the process release photons (light particles).
This process is similar to the discharge in a neon
lamp (fig. 4).
Any particular color of the aurora depends on
a specific atmospheric gas and its electrical
state, and on the energy of the particle that hits
the atmospheric gas. Atomic oxygen is respon-
sible for the two main colors of green (wave-
length of 557.7 nm) and red (630.0 nm).
Variations on the Sun
The Sun is a highly variable star that changes on
time scales of hours to hundreds of years. The
interplanetary magnetic field direction and so-
lar wind speed and density are driven by the ac-
tivity on the Sun. They can change drastically
and influence the geomagnetic activity. As geo-
magnetic activity increases, the southern edge
of the aurora borealis usually moves to lower
latitudes. Similarly, solar mass ejections coin-
The Cause of the Aurora
Excited oxygen /p\
Red Light ' y
(600-700 nm) X
Figure 4. — Fast electrons from space travel along
the magnetic field and strike oxygen atoms or nitro-
gen molecules in the atmosphere. Energy from an
atom or molecule excited by fast electrons is re-
leased as different colors of light.
cide with larger auroral ovals. If the interplane-
tary magnetic field is in the opposite direction
of the Earth's magnetic field, there can be in-
creased energy flow into the magnetosphere
and thus, increased energy flow into the polar
regions of the Earth. This will result in an inten-
sification of the auroral displays.
Disturbances in the Earth's magnetosphere are
called geomagnetic storms. These, in turn, can
produce sudden changes in the brightness and
motion of the aurora called auroral substorms.
The magnetic fluctuations of these storms and
substorms may cause surges in electric power
lines and occasional equipment failures in the
power grid, resulting in wide-spread power out-
ages. They can also impact the performance of
satellite-to-ground communications and naviga-
tion systems. Magnetospheric storms can last
several hours or even days, and auroral sub-
storms can occur several times a day. Each sub-
storm can deliver several hundred terajoules of
energy — as much as the electrical energy con-
sumed in the entire United States over 10 hours.
Measuring the Geomagnetic Field
The geomagnetic field can be measured with
instruments called magnetometers. Data from
many magnetometers allow observers to track
the current state of the geomagnetic conditions.
The magnetometer data are often given in the
form of 3 -hourly indices that give a quantitative
measure of the level of geomagnetic activity.
One such index is called the K-index.
The K-index value ranges from to 9 and is di-
rectly related to the amount of fluctuation (rela-
tive to a quiet day) in the geomagnetic field
over a 3-hour interval. The higher the K-index
value, the more likely it is that an aurora will oc-
cur. The K-index is also, necessarily, tied to a
specific observatory location. For locations
where there are no observatories, one can only
estimate what the local K-index would be by
looking at data from the nearest observatory. A
global average of auroral activity is converted
to the Kp index. This index is available on a dai-
ly basis over the World Wide Web.
There are many sources for beautiful pictures
of the aurora. A few are suggested here, and a
longer list can be obtained from Space Environ-
Gift Shop, Geophysical Institute
University of Alaska
Fairbanks, AK 99775-0800
Astronomical Society of the Pacific
390 Ashton Ave.
San Francisco, CA 94112
National Geophysical Data Center
NOAA, E/GC2 Dept. 945
325 Broadway, CO 80303-3328
Remember also that many resources exist on
the World Wide Web. Space Environment Cen-
Written by Larry Combs and Rodney Viereck, 1996
ter Web page is a good starting point for some
of these resources:
Many beautiful pictures have been published in
articles and books. Here are two:
Majestic Lights, Robert H. Eather, American
Geophysical Union, Washington, D.C., 1980
Aurora Borealis, S.-I. Akasofu, Alaska Geographic
Society, vol. 6, no. 2, Anchorage, AK, 1979.
Do-It- Yourself Resources
You can take your own photos of the aurora if
you are in a location that permits good viewing
of the event. This is an excellent article that will
help you to do that:
"How to Photograph the Northern Lights," Floyd
— :on, Alaska magazine, November, 1973.